1. Field of the Invention
The present invention relates to a neuromuscular magnetic stimulation device.
2. Description of the Prior Art
In the field of medical diagnosis and therapy, magnetic stimulation devices serve for the magnetic stimulation of nerve fibers and muscular tissue. Compared to electrical stimulation by means of stimulating current, an advantage of magnetic impulse stimulation is the low amount of pain associated with the stimulation, since higher current densities do not arise in the region of the pain receptors of the skin. Another advantage of magnetic stimulation is the greater penetration ability, it being possible to excite deeper tissue as well, particularly deeper nerve fibers.
The book "Neuro- and Sinnesphysiologie" (R. F. Schmidt, pub., 2.sup.nd ed, 1995: chapters 2 and 3) contains a detailed description of neurophysiological processes. The nervous system coordinates the activities of the various organs and reactions of the body to the environment, for example. This occurs primarily by an alteration of the potential of nerve cells. All cells have a resting potential. At the resting potential, all membrane currents of a cell exist in balance. If the potential is depolarized by an additional membrane current, which enters the cell by means of an external influence, for example, this is associated with a potential variation, known as an action potential. This depolarizing membrane current is also called stimulation. The triggering potential for an action potential is called the threshold. At the threshold, the balance of the membrane currents changes. For a short time, additional membrane currents arise which depolarize the membrane. This state is also called excitation. An action potential is associated with an action.
For example, every contraction of a muscle fiber is accompanied by an action potential in the muscle fiber, and every reaction of a sensory cell to a sensory stimulus is transmitted by action potentials.
European Application 0 182 160 teaches a device for generating electromagnetic impulses with a semi-circular shape, which serves particularly for the promotion of the microcirculation of blood in the region of the hair roots and the skin, for example, to counteract hair loss. To this end, a diode rectifier bridge is connected to an alternating voltage transformer in a Graetz circuit which supplies a coil that generates pulses.
German OS 36 07 235 teaches a device for generating unipolar air ions and electromagnetic pulse fields for reducing human reaction time while simultaneously increasing attentiveness. To generate the electromagnetic impulse fields, a frequency generator with a decoupling amplifier connected downstream, and a coil which generates the pulse field, are connected to a voltage source.
German OS 41 32 428 teaches a magnetotherapy device for magnetotherapeutic treatment. To generate a pulsed magnetic field, an unstable multivibrator is connected to a battery, this multi-vibrator feeding two cylindrical coils with iron cores. The device is constructed as a pocket device.
U.S. Pat. No. 5,743,844 teaches a device for therapy by means of pulsing electromagnetic fields for the promotion of the healing of bone and body tissues, particularly in the form of a battery-powered device which can be carried on the body. A coil which generates the magnetic field is supplied from two voltage sources of different voltage levels via a specific circuit which contains two field effect transistors and two capacitors as basic elements. This circuit thus has a fixed pulse-pause time ratio.
The devices described in the above referenced patents are all designed such that the magnetic pulse fields, or alternating fields, which they generate act on the human body below the threshold for triggering action potentials. The effects in the human body that can be so achieved part very diffuse and scientifically controversial. Magnetic stimulation devices which purposefully trigger action potentials, particularly in deeper neuromuscular tissue, are another category of devices altogether. Not only is the use and therapeutic effect of these devices different, but also the electrical powers to be applied for this purpose are many times greater, which is exhibited in correspondingly high current and voltage values. Due to their overall low-voltage and low-current design, the devices described in the above referenced patents are not suited to this purpose.
A magnetic stimulation device for triggering action potentials even in deeper neuromuscular tissue is described in the essay "Entwickling, Optimierung and Erprobung neuer Gerate fur die magnetomotorische Stimulation von Nervenfasern," (M. Schmid, T. Weyh and B. -U. Meyer, Biomedizinische Technik 38,1993:317-324). This device has a stimulation coil which, together with a high-voltage capacitor, forms a parallel resonant circuit, i.e. which functions as a resonant circuit. The high-voltage capacitor is charged by a controllable network part and thereby accumulates the necessary pulse energy for the emission of a stimulation impulse. For this purpose the controllable network part is connected to the high-voltage capacitor with the terminals used for charging.
A thyristor is inserted, as an electronic switch, into the circuit of the high-voltage capacitor, which closes via the stimulation coil. While the high-voltage capacitor is charged by the controllable network part in preparation for the pulse emission, the thyristor remains open, and the parallel resonant circuit is interrupted. When the charge voltage reaches the value desired by the user, a charging switch belonging to the charging connection disconnects the high-voltage capacitor from the controllable network part, and the stimulation pulse is triggered by a firing of the thyristor. The high-voltage capacitor discharges via the stimulation coil. When the high-voltage capacitor has been completely discharged, the direction of the energy flow reverses. It would now be possible for the high-voltage capacitor to be charged with a reversed polarity, but this is avoided by an unbiased circuit branch arranged parallel to the high-voltage capacitor as an additional current path.
The unbiased branch contained an unbiased diode and a resistor connected in series with this diode.
In the unbiased branch, the energy ring-down from the stimulation coil is largely absorbed. The unbiased diode assures that the unbiased branch takes over the current only when the recovery (negative polarity energy flow) from the stimulation coil replaces the positive polarity discharging process of the high-voltage capacitor (commutation). Ultimately, the high-voltage capacitor, which nevertheless is slightly charged given reversed polarity, also delivers its energy to the resistor.
The resonant frequency of the parallel resonant circuit formed by the stimulation coil and the high-voltage capacitor is determined by the selection of the capacitance of the high-voltage capacitor and the inductance of the stimulation coil, and lies in the range of 1 to 3 kHz. If the capacitance of the high-voltage capacitor is varied, then the resonant frequency of the parallel resonant circuit and thus the rate of the current rise in the stimulation coil can be changed. The stimulation intensity is determined by the initial voltage at the high-voltage capacitor. As an additional parameter, the repetition rate, which lies in the region of about 10 Hz, can also be set.
German OS 196 07 704 also teaches a device for the magnetic excitation of neuromuscular tissue. The known device has an excitation coil (stimulation coil) which, together with a storage capacitor (high-voltage capacitor), forms a parallel resonant circuit, i.e. which likewise functions as a resonant circuit. In this device also, only resonant frequencies in the range of 1 to 3 kz can be realized.